Breakups are a whirlwind of emotions. There's a lot of uncertainty, sadness, fear, disappointment and depending on the relationship, there's sometimes anger (or even relief). Whether your relationship ended well or on a bad note, these are things that your ex is definitely thinking right now.
"Wow, I didn't realize how much we did together."
"Everything reminds me of you."
"Was breaking up the right thing to do?"
"What if you start dating someone else right away?"